Switching regulator with network to reduce turnon power losses in the switching transistor

ABSTRACT

Power dissipation losses occur during the turnon transition period of the switching transistor of a switching regulator. These losses are due to the voltage appearing across the switching transistor during current conduction within the turnon transition period. A turnon loss reduction network is added to the switching regulator to reduce the power dissipation inthe switching transistor during turnon. This turnon loss reduction network counteracts the voltage normally appearing across the switching transistor and hence reduces the power dissipation therein due to this voltage. A lossless charging arrangement is included in the regulator to reduce the power dissipation that occurs within the turnon loss reduction network.

afienfi 19 Calkin et al. 14 1 July 10, 197

[54] SWITCHING REGULATOR WITH 3,566,253 2/1971 O'Neill 323/D1G. 1NETWORK o REDUCE TURNON POWER 3,641,422 2/1972 Farnsworth et al 323/D1G.1 3,621,372 11/1971 Paine 323/DIG. 1 LOSSES IN THE SWITCHING TRANSISTOR3,663,949 5/1972 Froeschle 323/010. 1 [7 5] Inventors: Edwin TheodoreCalkin, Parsippany;

Billy Harold Hamilton, Sf Primary Examiner-A. D. Pellinen Carl La P0118,Livingston, Att0meyW. L. Keefauver et al.

0 [73] Assignee: Bell Telephone Laboratories, Inc., [57] ABSTRACT MurrayPower dissipation losses occur during the turnon transi- 2 Filed; Man22, 7 tion period of the switching transistor of a switching regulator.These losses are due to the voltage appearing [21] PP N04 236,852 acrossthe switching transistor during current conduction within the turnontransition period. A turnon loss 52 US. Cl 323/17, 321/2, 323/9,reduction network i added to the Switehing regulator 323/22 T, 323/DIG 1to reduce the power dissipation inthe switching transis- [51] Int. Cl.H02m 3/32 during tumon- T turnon reduction network 58 Field of Search321/2; 323/9, 17, counteracts the voltage normally appearing eerees the323/22 T, DIG 1 switching transistor and hence reduces the powerdissipation therein due to this voltage. A lossless charging 5References Cited arrangement is included in the regulator to reduce theUNITED STATES PATENTS power dissipation that occurs within the turnonloss reduction network. 3,383,584 5/1968 Atherton 323/DIG. 1 3,600,6668/1971 Gliever 323/DIG. 1 3 Claims, 8 Drawing Figures BASE DRIVE CONTROLPATENIE JUL 1 0192s SHEEI 1 0F 2 F/G. (PRIOR ART) ij/IOI l 0 |oe l IOB glog T I A I BASE DRIVE CONTROL BASE DRIVE CONTROL PATENIEU JUL I 01915saw 2 ur 2 c I FIG. 48

BASE DRIVE CONTROL H6. 6

6 22 606 620 "5m 02 I 0 f' 604 I W are BAS E DRIVE CIRCUIT CONTROLSWITCHING REGULATOR WITH NETWORK TO REDUCE TURNON POWER LOSSES IN THESWITCHING TRANSISTOR BACKGROUND OF THE INVENTION This invention relatesto power supplies and more specifically to switching mode regulators. Itis specifically concerned with the reduction of switching lossesoccurring in the switching device of the switching regulator.

A typical switching mode regulator such as is shown in FIG. 1 utilizes atransistor switch 110 to intermittently couple a voltage source 101 toan output load 109. The voltage level at the output load 109 isdetermined by the duration that the switching transistor 110 conductswithin each cycle of operation. The switching of the switchingtransistor 110 is controlled by a base drive control circuit 105. Thisbase drive control circuit 105 may comprise circuitry either external orinternal to the switching regulator. It may be responsive to either thecurrent or voltage output of the regulator or to both of thesequantities. Regulation feedback circuits to accomplish this switchingcontrol are well known to those skilled in the power supply art and,hence, it is not believed necessary to describe it in detail.

The current transmitted by the switching transistor 110 is coupled tothe load 109 via a flyback inductor 107. The current inertiacharacteristics of the flyback inductor 107 stabilizes the outputcurrent of the regulator. When the transistor switch 110 is biasednonconducting, the stored energy in the flyback inductor 107 suppliesthe current to the load 109. It provides this current to the load 109via a flyback diode 106. A capacitor 108 shunting the output load alsosupplies current by discharging during this interval.

The transistor switch 110, when conducting, is operated in a saturatedmode. Since the voltage drop across the transistor 110 is very smallduring saturation, very little power is dissipated therein. Power,however, is dissipated in the transistor 1 during the transition periodsfrom its nonconducting to its conducting state. The current and voltageresponse of the switching transistor 1 10 during this turnon transitionperiod is shown by the current and voltage waveforms of FIG. 2A. As isapparent from the waveforms shown in FIG. 2A, as the transistor switch110 is biased into its conducting state, the collector current i slowlyincreases from a zero value to the saturated current value I in a linearramp type fashion. As is apparent from the voltage waveform v thevoltage drop across the collectoremitter path of transistor 1 10 remainsat nearly the cutoff value V until the current reaches the saturatedvalue I. The power loss in the switching transistor 110 during the timewhen the current is increasing from zero to its saturated value I maydissipate a considerable portion of the input power applied to theregulator. The current and voltage response of the transistor 110 may bereadily ascertained by reference to FIG. 2B which indicates that thevoltage across the transistor 110 remains at approximately its maximumvalue V until the saturated current level I in the collector emitterpath of the transistor is achieved. This power loss during the turnonmay require the use of a switching transistor having a very high powerdissipation capacity together with a large heat sink device.

It is, therefore, an object of the invention to significantly reduceturnon loss in the switching transistor of a switching regulator.

It is also an object to reduce power dissipation losses generally in aswitching regulator.

It is yet another object to improve the efficiency of operation ofswitching transistors in a switching regula tor and permit theirutilization at their rated capacity.

SUMMARY OF THE INVENTION Therefore, in accord with the invention, aturnon loss reduction network is included in a switching regulator toreduce the turnon power losses in its switching transistor. The turnonloss reduction network includes a compensating inductor which isconnected to the flyback diode of the regulator. During the turnontransition of the switching transistor, while the flyback diode is stillforward biased, the compensating inductor generates a voltage which isequal to the input voltage supplied to the regulator. This inducedvoltage is utilized to absorb the input voltage rather than permittingit to appear across the terminals of the switching transistor during'theturnon transition period thereby reducing energy dissipation in thetransistor due to current flow during this transition time. The energystored in the compensating inductor, just prior to the turnon of theswitching transistor, is discharged into the output load during theturnon transition period of the switching transistor.

The chargingof the compensating inductor prior to this turnon lossreduction operation consumes energy since the compensating inductor isdissipatedly charged. Added circuitry is included in this turnon lossreduction network to eliminate the charging losses by losslesslycharging the compensating inductor. A feature of this invention is themonitoring of current flowing through this lossless charging path toprovide over current protection of the switching regulator output.

BRIEF DESCRIPTION OF THE DRAWINGS Many additional features, advantagesand other objects of the invention will become apparent uponconsideration of the following detailed description of a specificswitching type regulator loss reduction network utilizing the principlesof the invention. The following description is to be taken inconjunction with the attached drawings in which:

FIG. 1 is a schematic of a switching regulator of the prior art which isdescribed hereinabove;

FIGS. 2A and 2B disclose signal waveforms and switching loci toillustrate the signal response characteristic of the switchingtransistor of the regulator shown in FIG. 1 during the turnon transitionperiod;

FIG. 3 is a schematic of a switching regulator including a turnon lossreduction network to reduce the turnon losses in the switchingtransistor of a switching regulator;

FIGS. 4A and 48 comprise signal waveforms and switching loci toillustrate the signal response characteristics of the switchingtransistor of the switching regulator shown in FIG. 3 during the turnontransition pe-- riod;

FIG. 5 is a schematic of a switching regulator with a lossless chargingarrangement to charge the turnon loss reduction network; and

FIG. 6 is a schematic of a switching regulator utilizing thecharacteristics of the turnon loss reduction network to provideovercurrent protection.

DETAILED DESCRIPTION The switching regulator disclosed in FIG. 3intermit tently couples a voltage source 301 to an output load 309 byintermittently switching the switching transistor 310 into alternateconducting and nonconducting states. The circuit operates identically tothe circuit described above with reference to FIG. 1 and hence need notbe described in detail. The tumon'losses in the switching transistor310, however, are significantly reduced by a turnon loss reductionnetwork 300 included in the circuit. The turnon loss reduction network300 comprises a compensating inductor 315 connected in series with theflyback diode 306. The compensating inductor 315 interconnects theflyback diode 306 to a junction 325 at which the collector electrode 311is connected to the flyback inductor 307. A charging network comprisinga series-connected diode 3 17 and charging resistor 316 shunts thecompensating inductor 315.

An understanding of the turnon loss reduction network 300 may be readilyacquired by describing its operation during the turnon transition periodof the switching transistor 310. Just prior to the turnon of theswitching transistor 310 a voltage approximately equal to the inputvoltage of the source 301 exists across the collector-emitter electrodesof the switching transistor 310. The current to the output load 309 issustained by the flyback inductor 307. This current flows through thecompensating inductor 315 and the diode 306. As the base drive control305 biases the switching transistor 310 into conduction, the current inthe flyback diode 306 begins to decrease. Hence, the current flowingthrough the compensating inductor 315 begins to decrease. Thecompensating inductor 315 attempts to sustain the current at the samelevel which had been flowing through it just an instant before. Hence alarge voltage is induced across the inductor 315. This voltage is equalin magnitude and opposite in phase to the voltage of the input voltagesource 301. It is apparent that at this pointonly a very low voltageappears across the collector-emitter terminals of the switchingtransistor 310. Hence the collector-emitter voltage of transistor 3K0 isreduced to a very low value during the time period when current beginsto flow through the transistor 310 and increases to its saturated valueI.

The foregoing may be readily ascertained by reference to FIG. 4A whereit is apparent that the collectoremitter voltage v of the switchingtransistor 310 drops to a very low value as current conduction begins.The colector-emitter voltage v drops almost immediately to a very lowvalue and the collector current i of the switching transistor 310increases to its saturated value I with a very low voltage dropappearing across the collector-emitter terminals of the switchingtransistor 310. The switching loci is shown in FIG. 48 wherein the powerdissipated during switching is represented by the area under the curvelocus in FIG. 4B. The improved efficiency becomes apparent when thisarea is compared with the area under the curve locus in FIG. 2B.

- Energy is stored in the inductor 315 during the turnoff transistionperiod of the switching transistor 310. This energy is supplied by acharging voltage which appears as a result of a charging current flowthrough the diode 317 and the charging resistor 316 as the transistor310 turns off. The energy dissipated in the charging resistor 316 duringturnoff may represent a considerable portion of the power saved duringthe turnon transition period in transistor 310. In instances wherecircuit efficiency is very important and where heat dissipationrepresents a problem, it is desirable to eliminate this power loss inthe charging resistor 316.

This power dissipation in the charging resistor 3116 may be eliminatedby eliminating the charging resistor 316 and substituting in its place alossless charging network. In FIG. 5 the turnon loss reduction networkis modified to permit lossless charging of the compensating inductor515. The lossless charging network comprises an additional charginginductor winding 520 which is wound on the core of the inductor 515.This charging inductor winding 520 is directly connected across theinput voltage source 501 through diode 526. As current flows through theflyback diode 506, the current flowing in the charging inductor winding520 is initially at a value proportional to the output load current.This initial value is dependent upon the turns ratio between thecharging inductor winding 520 and the winding of the compensatinginductor 515.

The compensating inductor 515 is charged by the charging inductorwinding 520 at a constant voltage which is equal to the input voltage ofthe voltage source 501. This permits a rapid and uniform charging of thecompensating inductor 515. A diode 526 is connected in series with thecharging inductor winding 520 and poled to prevent reverse current flowthrough winding 520 due to the input voltage source 501.

The current flowing through the charging inductor winding 520, asindicated above, is proportional to the load current at the instant ofturnon of the switching transistor 510. An overcurrent protectioncircuit is shown in FIG. 6 to utilize this proportional current tocontrol an overcurrent protection circuit. This proportional current isused to operate a protection circuit 603 to prevent the occurrence ofovercurrent in the switching transistor 610. To operate this overcurrentprotection circuit 603 a current sensing resistor 604 is placed inseries with the charging inductor winding 620. The overcurrentprotection circuit 603 has a current level detector which is connectedin shunt with'the resistor 604. The protection circuit 603 in responseto the occurrence of an overcurrent applies a signal to the base drivecontrol 605 to turn off the switching transistor 610.

The switching regulator in FIG. 6 includes a series connected capacitor602 and diode 622 which shunt the flyback diode 606 and the inductor615. This circuit comprises a turnoff loss reduction network which isdescribed in detail in applicants copending application, filed Mar. 22,1972 Ser. No. 236,833 and assigned to the same assignee. The turnoffloss reduction network is disclosed herein to describe its function incountering certain adverse effects of the turnon loss reduction networkduring the turnoff of the switching transistor 610.

Referring again to FIG. 5, as the switching transistor 510 turns off,the induced voltage of the flyback inductor 507 attempts to sustain theload current which the switching transistor 510 was conducting. Sincethe switching transistor 510 is now becoming nonconducting, this loadcurrent must flow through the compensating inductor 515 and the flybackdiode 506 to the load. The compensating inductor 515 generates aninduced voltage which opposes this current flow and which is in phasewith the voltage of the input voltage source 501.

The induced voltage is limited to a value proportional to the inputvoltage source and the turns ratio between the compensating inductor 515and the charging inductor winding 520. The combined voltage of the inputvoltage source 501 and the compensating inductor 515 applies a voltageacross the collector-emitter path of the switching transistor 510 whichexceeds the voltage of the input source. This voltage which appearsduring turnoff while collector current is still decreasing causes apower dissipation which may overstress the switching transistor 510. Theaddition of the capacitor 602 to the circuit of FIG. 6 constrains thecollector voltage of the switching transistor 610 to increase graduallyas it turns off. This is due to the linearly increasing dischargecurrent of the capacitor 602 during the turnoff transition time of thetransistor 610. The resultant gradual rise of the collector voltage oftransistor 610 as its collector current decreases results in a greatlyreduced power dissipation in the switching transistor 610.

What is claimed is:

1. A switching type regulator comprising input terminals, outputterminals, a first series connection including a switching transistorand a flyback inductor coupling one of said input terminals to one ofsaid output terminals, a common junction coupling said transistor andsaid flyback inductor, a second series connection including a secondinductor and a flyback diode, said second series connection beingconnected to said junction coupling said switching transistor and saidflyback inductor and to the other ones of said input and outputterminals, wherein the improvement comprises, a third inductor windingand a second diode connected as a third series connection, said thirdseries connection connected in shunt across said input terminals, saidthird inductor winding being magnetically coupled to said secondinductor and said second diode being poled to conduct in response toinduced signals in said third inductor when said flyback diode isconducting.

2. A switching regulator as defined in claim 1 further including afourth series connection including a third diode and a capacitorshunting said second series connection wherein said capacitor dischargesduring the turnoff of said switching transistor to limit the voltagerise across said switching transistor.

3. A regulating circuit including an input circuit, a switchingtransistor intermittently driven into alternately conducting andnonconducting states, an output circuit including a flyback inductor anda flyback diode to supply current during the nonconducting state of saidswitching transistor, means to eliminate a voltage drop across saidswitching transistor during its transition from a nonconducting to aconducting state including a compensating inductor connected betweensaid flyback diode and said flyback inductor, the connection of saidflyback inductor and said compensating inductor being connected to thecollector of said switching transistor, wherein the improvementcomprises, means to store energy in said compensating inductor duringthe nonconducting state of said switching transistor comprising acharging inductor winding shunting said input circuit and magneticallycoupled to said compensating inductor, whereby a reverse voltage inducedin said compensating inductor as said switching transistor switches froma nonconducting to a conducting state counteracts the voltage dropacross said switching transistor, and said charging inductor permitslossless charging of said compensating inductor. of said switchingtransistor, wherein the improvement comprises, means to store energy insaid compensating inductor during the nonconducting state of saidswitching transistor comprising a charging inductor winding shuntingsaid input circuit and magnetically coupled to said compensatinginductor, whereby a reverse voltage induced in said compensatinginductor as said switching transistor switches from a nonconducting to aconducting state counteracts the voltage drops across said switchingtransistor, and said charging inductor permits lossless charging of saidcompensating inductor.

UNITE STATES PATENT OFFICE '(IERTIFICATE OF EORRECTION Patent No. 3,,444 Dated Julv 10. 1973 Ipventofls) EDWIN THEODORE CALKIN, ET. AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In the Abstract, line 7, "inthe" should read in the Column 6, line 28,after the period, "of

said" through line 39 should be cancelled.

Signed and sealed this 22nd day of January 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Q ActingCommissioner of Patents FORM Fae-105 (10-69) 1 USCOMM-DC 60376-P69 U.5.GOVERNMENT PRINTING OFFICE {959 0-866-334.

h 1" UNTTEE STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pater zt No.4 3,745 .444 Dated Julv 10. 1973 -(Q EDWIN THEODORE CALKIN, ET. AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In the Abstract, line 7, "inthe" should read in the Column 6, line 28,after the period, "of

said" through line 39 should be cancelled.

Signed and sealed this 22nd day of Januery 1974.

(SEAL) Attest: N

EDWARD M. FLETCHER,JR. RENE D TEGTMEYER Attesting Officer g ActingCommissioner of Patents FORM PQ-15 uscoMM-Dc scan-eon U. 5. GOVERNMENTPRINTING OFFICE "Bl 0-86G-33L

1. A switching type regulator comprising input terminals, outputterminals, a first series connection including a switching transistorand a flyback inductor coupling one of said input terminals to one ofsaid output terminals, a common junction coupling said transistor andsaid flyback inductor, a second series connection including a secondinductor and a flyback diode, said second series connection beingconnected to said junction coupling said switching transistor and saidflyback inductor and to the other ones of said input and outputterminals, wherein the improvement comprises, a third inductor windingand a second diode connected as a third series connection, said thirdseries connection connected in shunt across said input terminals, saidthird inductor winding being magnetically coupled to said secondinductor and said second diode being poled to conduct in response toinduced signals in said third inductor when said flyback diode isconducting.
 2. A switching regulator as defined in claim 1 furtherincluding a fourth series connection including a third diode and acapacitor shunting said second series connection wherein said capacitordischarges during the turnoff of said switching transistor to limit thevoltage rise across said switching transistor.
 3. A regulating circuitincluding an input circuit, a switching transistor intermittently driveninto alternately conducting and nonconducting states, an output circuitincluding a flyback inductor and a flyback diode to supply currentduring the nonconducting state of said switching transistor, means toeliminate a voltage drop across said switching transistor during itstransition from a nonconducting to a conducting state including acompensating inductor connected between said flyback diode and saidflyback inductor, the connection of said flyback inductor and saidcompensating inductor being connected to the collector of said switchingtransistor, wherein the improvement comprises, means to store energy insaid compensating inductor during the nonconducting state of saidswitching transistor comprising a charging inductor winding shuntingsaid input circuit and magnetically coupled to said compensatinginductor, whereby a reverse voltage induced in said compensatinginductor as said switching transistor switches from a nonconducting to aconducting state counteracts the voltage drop across said switchingtransistor, and said charging inductor permits lossless charging of saidcompensating inductor. of said switching transistor, wherein theimprovement comprises, means to store energy in said compensatinginductor during the nonconducting state of said switching transistorcomprising a charging inductor winding shunting said input circuit andmagnetically coupled to said compensating inductor, whereby a reversevoltage induced in said compensating inductor as said switchingtransistor switches from a nonconducting to a conducting statecounteracts the voltage drops across said switching transistor, and saidcharging inductor permits lossless charging of said compensatinginductor.